Retrodirective phased array antenna for a spacecraft



Mrch 10, 1970 J. o. KIESLING 3,500,411

RETRODIRECTIVE PHASED ARRAY ANTENNA FOR A SPACECRAFT Filed April 26, 1967 Y 2 Sheets-Sheet 1 n oTHE L 1 ELEMENTs E 3 ANTENNA FEED5 FOP.

8? ANTENNA Feeos FoR ANTENNA 86 FE F'OIL 5E ANTENNA F5505 Fol INVENTOR 2" JouN DoNnw I |E5L|N6 BY MM 91431;:

A T TORNE Y March 10, 1970 I J, K s 3,500,411

RETRODIRECTIVE PHASED ARRAY ANTENNA FOR A SPACECRAFT Filed April 26. 1967 2 Sheets-Sheet 2 Mom: 88 GRouPM Gnoupqo 620cm):

Y v/vvv vw vv vw W Y '95 I I, e1 l I I l l I q I I I I I l l I I rpm/5 I I I I I I l I I I AM I I I I I l I I/PZEXER l I l I I I I l I D 116 I INVENTOR JoI-Iu Domaw KIE5LIN6 BY Ida/we ho7 United States Patent 3,500,411 RETRODIRECTIVE PHASED ARRAY ANTENNA FOR A SPACECRAFT John Donald Kiesling, East Brunswick, N.J., assignor to RCA Corporation, a corporation of Delaware Filed Apr. 26, 1967, Ser. No. 633,907 Int. Cl. H04b 7/02 U.S. Cl. 343-100 ABSTRACT OF THE DISCLOSURE 4 Claims BACKGROUND OF THE INVENTION With the continued use of satellites as a vital link in future and present communication systems, there is a need for antenna systems which have extreme mechanical simplicity together with the capability of providing electronic scanning with multiple beams to cover desired areas on the surface of the earth or other celestial bodies. An antenna employed aboard a satellite should be compatible with solid state circuits in order to provide improved satellite performance evidenced by lightweight reliable equipment and simple structural design. Due to the potentially large area that satellites can accomodate in providing communications coverage thereto, there is a need for an antenna capable of providing multiple beams to accomodate multiple locations which may have different frequency allocations or require different transmitted information. Prior art antenna systems for satellites encompass certain desirable features in their design but there still exists a need for an antenna which is capable of multiple beam generation while providing individual scan capabilities for each beam.

It is therefore an object of the present invention to provide an improved antenna system for use aboard a satellite.

Another object is to provide a satellite antenna system capable of producing multiple beams.

Still a further object is to provide an improved satellite antenna system capable of producing multiple beams while allowing each beam to be electronically scanned over a relatively large angle.

SUMMARY OF THE INVENTION These and other objects of the present invention are accomplished in one embodiment by using a reflector, which reflector may be parabolic or spherical in shape. The reflector uses at its focal point or focal region a retrodirective phased array. The retrodirective phased array acts, by suitable positioning within the reflector, as a primary feed. In this manner the energy reflected by the reflector is distributed in amplitude and phase over the aperture of the retrodirective feed. The arrangement is such that the retrodirective feed can regenerate a received signal amplitude distribution, and the conjugate phase distribution, at a dilferent frequency than that of the received signal. In this manner the retransmitted signal is caused to follow a propagation path determined by the received sig nal. A routing circuit can be coupled between the various elements of the retrodirective array comprising the primary feed to give the combination a multiple beam capa- "ice bility. This characteristic can be further determined and controlled by a series of ground stations or beacons distributed upon the surface of the earth or some other suitable body.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURES 1A to 1C are perspective views of a reflector and a feed according to this invention.

FIGURE 2 is a partial side cross sectional view of an antenna reflector showing the positioning of the primary feed and associated electronics.

FIGURE 3 is a pictorial view of a satellite in relation to ground stations operating in accordance with this invention.

FIGURE 4 is a view presenting an example of the geometric arrangement at the focal region of a reflector type antenna as used in this invention.

FIGURE 5 is a partial block and schematic diagram showing trafiic routing circuitry associated with an antenna array according to this invention.

DESCRIPTION If reference is made to FIGURE 1A, there is shown a front view of a reflector 81. The reflector 81 is fabricated from a conducting material, for example, a metal such as aluminum or magnesim, and may be parabolic or hemispherical in shape. The shape of the reflector 81 can be determined according to the particular application. Shown at or near the geometric center or focal point of reflector 81 is an antenna feed 82. The antenna feed 82 is a retrodirective phased array to be described and is preferably located near the focal region of the reflector 81 such that the energy reflected by the reflector 81 is distributed in amplitude and phase over the aperture of the antenna feed 82. Retrodirective antenna arrays have been described in the art and their conditions for operation set forth. An example of such an array is described in U.S. Patent 2,908,202 to L. C. Van Atta, issued Oct. 6, 1959. The array described in the above patent is a passive device in contrast to the active conjugate phased array principle used herein. FIGURE 1B is included to further show the desired configuration, while FIGURE 1C indicates that the combination is capable of multiple beam generation as Will be described.

If reference is made to FIGURE 2, there is shown a side cross sectional view of the reflecting surface 81. Numeral 74 represents a housing for the electronic assembly which will be described in detail later on. The electronics within housing 74 includes active devices and circuits such as transponders with amplifiers and mixers, and so on. The electronics is coupled by suitable means to a plurality of antenna elements 70 through 73. Each individual element is mounted on the electronic housing 74 in a manner to provide the proper spacing for a retrodirective circuit. The electronic housing assembly 74 is positioned near the focal region or, in the case shown, the geometric center of reflector 81 by means of supporting or feed rods 75 and 76. The advantages of the configuration shown in the above described figures are considerable in that the shown antenna configuration is of mechanical simplicity, capable, for example, of being deployed on a satellite. Such an antenna is also capable of providing several beams simultaneously each of which may be electronically steered by an external beacon or ground station thus eliminating mechanical devices on the satellite and further providing the capability of scanning each beam over a relatively wide pointing angle.

FIGURE 2 also shows a ground station 77 situated on a celestial body as earth, whose antenna 84 emits a beam which is directed onto the surface of reflector 81, which may be aboard an orbiting satellite, in a manner to illuminate only a portion of the antenna elements, for example,

elements 70 and 71. The antenna elements all are coupled in a retrodirective configuration. Other antenna elements such as elements 72 and 73 not illuminated by the received beam in the array are operated to emit a beam in a direction towards ground station 77 or to emit a beam in a direction determined by ground station 77, for example, as that shown eminating from elements 72 and 73 to the antenna 83 of ground station 78. From the geometrical configuration of the reflector 81 in relation to its position with the ground stations 77 and 78, it can be seen that the angle of incidence of a beam received by the reflector 81 or transmited via the reflector 81 is a function of the positon of the reflector 81 and an appropriate ground station. By coupling the elements 70 through 73 as an active retrodirective array, one can further cause an angle change in any desired direction, thereby obtaining a broad scan angle capability. The elements 70 through 73 are representative only, as there may be many more, indicated by the dotted lines on the figure specifying n other elements. n can be any integer according to the system requirements. It is also noted that the antenna elements 70 through 73 are distributed on the housing 74 in a two dimensional array centered about the focal point or region of the spherical reflector 81. The location of housing 74 containing the elements 70 through 73 is such that the individual elements 70 through 73 in the array will be excited according to the manner the impinging radiation received by reflector 81 illuminates the array. Hence for various angles of incidence of an impinging beam or various different groups of these elements will be affected. It is also noted that the beam used to communicate may be a laser beam emanating from the ground station 77. Due to the spatial coherence properties of such a beam, a relatively small portion of the reflector 81 can thus be illuminated and cause a reflection of the beam to only certain of the elements 70 through 73 which can be light sensitive antenna elements, as photo diodes and so on, instead of those capable of responding to electromagnetic radiation not including light frequencies.

If reference is made to FIGURE 3, there is shown a satellite 85 which may be placed in a stationary orbit with respect to a celestial body 86 such as the earth. The satellite 85 has antenna 87 mounted thereon which antenna 87 can be similar to that shown in FIGURES 1A through 1C, namely, a reflector containing a retrodirective array as a feed. Also shown are ground stations 88, 89, 90 and 91 located at strategic points on the surface of the body 86 and in full view of the satellite 85. The satellite 85 is in communication with the ground stations 88 through 91 in the following manner. Ground station 88 transmits to the satellite 85 on a specified up link frequency F This signal transmitted from station 88 contains traflic information for stations 89, 90 and 91 and for a desired area about said stations. Similarly station 89 transmits the information to stations 88, 90 and 91 via satellite 85, and so on. In the satellite 85, the down link transmission to the ground stations 88 through 91 is broadcast on a frequency F In this manner each ground station 88 through 91 can use the same up link frequency F and the same down link frequency F However, other frequencies different for each station 88 through 91 can also be employed.

If reference is made to FIGURE 4, there is shown a front view of part of the antenna 87 of the satellite 85. The signal from the ground station 91 is caused to illuminate the cross hatched area near the focal region of the reflector 81 of antenna 87 which in turn illuminates the corresponding area of the feed. Specifically this is accomplished in the following manner. There is shown located at the center of the reflector 81 four circles designated as antenna feeds for stations 88 through 91. Each circle depicted pre-presents a plurality of individual antenna elements each of which may be a helix, dipole, turnstile or some other suitable antenna device. The important point is that the individual circles represent a plurality of elements arranged in a manner to form a retrodirective array. The individual antenna elements within each feed shown as a circle are those for instance shown in FIGURE 2 as 70 through 73. By causing only a portion or a group of these elements to transmit simultaneously or at a particular instant a certain predetermined area of the reflecting surface 81 is illuminated which results in a beam being reflected in only a given direction. In a like manner a signal transmitted from the earth or a ground station thereon, by impinging upon the reflector 81, will in turn cause a signal to be received by only certain ones of these antenna feeds and, hence, due to the retrodirective properties of the feeds will cause a beam to be redirected along a path determined by the impinging or received beam. A signal, for example, received from ground station 91 will illuminate the crossed hatched area shown on reflector 81 and from the reflector 81 will illuminate all the antenna elements associated with the feeds pertaining to station 91. The retrodirective circuitry, to be described, will cause a return beam to be directed towards a desired ground station. The active areas of the retrodirective feed to be illuminated may be selected prior to launch, as the ground stations 88 through 91 locations and the satellite 85s suborbital point are all known. The illumination areas on the reflector 81 surface will cause the excitation of the elements to receive the signal. These elements of course must be Within the scan angle of the reflector.

If reference is made to FIGURE 5, one embodiment of the routing equipment aboard the satellite 85 is shown. Each ground station of FIGURE 3 or any number of ground stations has a plurality of antenna elements associated with the satellite assigned thereto, which elements as discussed previously are arranged within the reflector and form a retrodirective array. Their location will cause a beam to be pointed at the ground station associated with these elements providing a received beam in the proper direction illuminates the appropriate area of the feeds of FIGURE 4 which contains these elements. In FIGURE 5 there is shown a plurality of antenna elements split up into four groups, respectively designated as groups 88 through 91, with each group corresponding to a ground station of FIGURE 3. The elements associated with each group 88 through 91 are arranged in a retrodirective array within a reflector 81 aboard a satellite as 85 of FIG. 3. As mentioned previously these elements are mounted in the general area of the focus point or region of a reflector or reflecting surface as 81 of FIGS. 1, 2 or 4. The elements also may be arranged at other locations within a reflector or reflecting surface for different applications. For the sake of simplicity one antenna element in each group is shown coupled to the associated circuitry to illustrate how a typical signal is routed. It is understood that each element in a group 88 through 91 has similar circuitry associated with it.

Antenna element 95 is an element in the retrodirective array and is also associated with ground station 88 and hence referenced as one in group 88 in FIGURE 5. Element 95 may be any radiating member such as a dipole, helical or turnstile antenna capable by itself of producing a fairly narrow radiation beam. The element 95 is coupled to one terminal of a diplexer 99. The function of the diplexer 99 is to allow the antenna element 95 to be used at both the transmitting and receiving frequencies while providing isolation between a receiver 100 and a transmitter 101. Many types of diplexers are suitable for use in block 99, for example, see Microwave Circuits by I. L. Altman, D. Van Nostrand Co., Inc., 1964, pages 351-358. An arm of the diplexer 99 is coupled to the re ceiver 100 which receiver is capable of responding to the frequency band transmitted by the ground station 88 of FIG. 3. The output of the receiver 100 is filtered to provide separate outputs for the information signals to be transmitted to ground stations 89 through 91 of FIGURE 3 via their associated array elements indicated as those in Groups 89 through 91. Hence, the receiver 100 is coupled to filters or mixer circuits 102 to 104, there being a separate one for each group. It is also understood that the receiver 100 is designed to provide a retrodirective function in response to the received signal. Such techniques are understood in the art. Filter 1.02 is coupled to block 111 associated with group 89. Block 111 is an exciter circuit which filters and amplifies the signal to be transmitted by the group 89 array. The output of exciter 111 is coupled to the input of a transmitting amplifier 112 that is coupled to an arm of diplexer 113 associated with group 89. In a similar manner the antenna element 96 associated with group 89 can be used for both transmission and reception of signals as can any other antenna element in the group or any other group.

As an example of the operation of the system shown in FIGURE 5, assume a satellites reflector, in which all the elements shown in FIG. 5 are positioned near the focal region, is illuminated by a beacon located at the surface of the earth as by one of the ground stations 88 depicted in FIGURE 3. The antenna elements associated with group 88 receive a signal in response to the satellite reflector being illuminated, as previously described. Element 95 which is in this group then responds to the signal which represents a portion of the wave front of the transmitted signal. The diplexer 99 causes this signal to be coupled to receiver 100 which amplifies it and splits and divides the power to mixers and matching devices 102 through 104. Devices 102 through 104 have coupled thereto respective oscillators 107 through 109 to provide signals for transmission back to earth at a frequency equal to the sum or difference of the output of the receiver 100 and the respective oscillators 107 through 109. Hence there is a signal provided via devices 102 and 104 for each of the groups 89,90 and 91 for transmission back to earth via the array associated with that group. When these respective arrays are excited, they illuminate the reflector which in turn reflects a beam pointed towards the desired ground station. If all these elements are activated then areas of the reflector will be illuminated simultaneously thereby pro ducing multiple beams, each one of which is associated with a different area on the earth or a different ground station. As previously stated, device 102 is coupled to exciter 111 of group 89 which exciter 111 amplifies and filters the signal. The output of exciter 111 is then coupled to a transmitting amplifier 112, also having retrodirective capabilities, which feeds the signal to the diplexer 113 which in turn causes the signal to be coupled to the antenna element 96 which transmits the signal to ground station 89. In a similar manner the device 103 is coupled to exciter 115 which is associatedwith group 90 and in the manner described excites transmitter amplifier 116 whose output is coupled to antenna element 97 associated with group 90 via diplexer 117 for transmission to ground station 90. It can be seen that the reception of a signal by elements 96 through 98 associated with groups 89 through 91 will cause a signal to be sent to exciter 105 associated with group 88. The signal is coupled via the exciter 105, the transmitting amplifier 101, and the diplexer 99 to the antenna element 95 to be transmitted at the desired down link frequency to ground station 88. Filters and impedance matching devices have not been shown since the fabrication and use of such devices are well known in the art. FIGURE 5 also shows, for example, that a single conversion oscillator 120, for signals transmitted by group 88, can be coupled to all the mixing or convertor circuits associated with the exciter 105, while there may be different oscillators as 107 through 109 which are respectively coupled to the mixers or convertors associated within a reflector aboard a satellite, or a plurality of reflectors, to automatically transmit a received signal as a plurality of directed beam patterns to provide communications coverage to areas located on the surface of the earth. Such areas may be determined by beacons or other ground stations suitably located.

What is claimed is:

1. An antenna for a satellite providing radio communication between said satellite and a remote station, comprising,

(a) a reflecting surface having a specified focal region,

(b). a plurality of individual antenna elements,

(0) means for mounting said antenna elements in close proximity to and spaced from one another at said reflecting surfaces focal region to form with said elements an active retrodirective phased array 0perable to redirect a radio signal in a direction determined by said remote station.

2. An antenna for a satellite providing radio communication between said satellite and a plurality of remote stations, comprising,

(a),a symmetrical reflecting surface having a specified focal region,

(b) a plurality of individual antenna selements, certain of said plurality of antenna elements forming a plurality of groups each one of which is associated with a particular one of said remote stations,

(c) means for mounting said antenna elements within said reflecting surface in a manner to position said groups in close proximity to said reflecting surfaces focal region, and

(d) routing means coupled to said elements in each of said groups to form therewith an active retrodirective array by which at least one of said groups is operable to direct a signal to or receive a signal from only one of said plurality of remote stations via said reflecting surface.

3. An antenna for a spacecraft providing radio communciation between said spacecraft and a plurality of remote stations, comprising,

(a) a reflecting surface having a specified focal region,

(b) a plurality of individual antenna elements, certain of which are arranged to form a group corresponding to one of said remote stations while others form separate groups corresponding to different ones of said remote stations,

(0) means for mounting said antenna elements within said reflecting surface in a manner to position each of said antenna element groups in close proximity with and separate from said reflecting surfaces focal region, and

(d) routing means coupled to each of said groups of elements to form therewith an active retrodirective phased array which operates each of said groups via said reflecting surface to redirect a signal only to its associated remote station when at least one other of said remote stations is transmitting.

4. An antenna system comprising (a) a reflecting surface,

(b) a plurality of antenna elements operated with said surface to form a retrodirective phased array responsive to a signal received at certain of said elements from one direction via said surface to transmit a further signal via others of said elements and said surface in a number of other, different directions.

References Cited UNITED STATES PATENTS 3,406,401 10/1968 Tillotson 343-400 RICHARD A. FARLEY, Primary Examiner T. H. TUBBESING, Assistant Examiner U.S. Cl. X.R. 343835 

